In 1968, Victor Veselago predicted that media with simultaneously negative dielectric permittivity and magnetic permeability would have an effective refractive index with a negative value. In the same work, he suggested that in such a material all physical phenomena directly related to phase velocity would be reversed. Specifically, the reversal of Snell's law, Cherenkov's radiation, and the Doppler effect were predicted. The characteristic of having simultaneously negative dielectric permittivity and magnetic permeability values creates several challenges in the design and fabrication of so-called negative index materials (NIMs). For example, although negative dielectric constants can be found in a variety of plasmonic materials (e.g., Ag, Au, Cu, and the like) or polar materials (e.g, SiC) materials, no naturally occurring substance is known to exhibit magnetism at sufficiently high frequencies (GHz to optical).
A variety of resonant structures have been proposed to obtain the nontrivial value of magnetic permeability desired in NIMs. However, the intrinsic presence of a resonance in these structures leads to extremely high (resonant) losses in the NIM. Another drawback of the resonant-based designs is their high sensitivity to fabrication defects.
Photonic crystals have also been shown to have a negative value of effective refractive index in the GHz frequency range. Because the typical size of inhomogeneity in these structures is on the order of the wavelength, however, the value of the effective dielectric constant strongly depends on the direction of light propagation. Such an anisotropy of the refraction index produces severe deterioration in image quality in a photonic crystal-based NIM lens, and makes subwavelength resolution in these systems practically unachievable. Furthermore, the crystalline nature of the material requires extremely high accuracy during the fabrication step, thus limiting the possible NIM structures to the GHz-frequency domain.
Accordingly, there is a need for new structures exhibiting a negative refractive index and methods for manufacturing such structures.